In the construction of many buildings, a finished ceiling, which is referred to as a drop ceiling, is spaced below a structural floor panel that is constructed of concrete, for example. The drop ceiling supports light fixtures and other ceiling-mounted items, while the space between the ceiling and the structural floor from which it is suspended serves as a return-air plenum for elements of heating and cooling systems as well as a convenient location for the installation of communications, computer and alarm system cables. It is not uncommon for these plenums to be continuous throughout the length and width of each floor.
When a fire occurs in an area between a floor and a drop ceiling thereabove, it may be contained by walls and other building elements which enclose that area. However, when and if the fire reaches the plenum, and if flammable material occupies the plenum, the fire can spread quickly through an entire story of the building and smoke can be conveyed through the plenum to adjacent areas. The fire could travel along the length of communications cables which are installed in the plenum and which comprise a plurality of conductors individually insulated with a plastic material and enclosed in a jacket comprising a plastic material.
Because of the possibility of such a flame spread and smoke evolution, particularly when aided by flammable insulation of cables, the 1975 edition of the National Electric Code (NEC) prohibited the use of electrical cables in plenums unless they were enclosed in metal conduits. Since rigid metal conduits are difficult to route in plenums congested with other items, a rearrangement of office telephones which in some companies has almost become an annual event, is extremely expensive. However, the code permits certain exceptions to this cost prohibitive requirement. For example, flame-resistant, low smoke producing cables without metallic conduit would be permitted provided that such cables were tested and approved by an authority such as the well known Underwriters Laboratories. What is needed is a cable for use in buildings which is relatively inexpensive to manufacture, but which meets the NEC requirements for flame retardance and smoke evolution, and which has excellent mechanical properties, particularly mechanical flexibility.
In the marketplace, cable which comprises a core having a paper core wrap and enclosed in a relatively thick metallic shield is available, but it is relatively inflexible and somewhat difficult to maneuver in plenums. Moreover, care must be taken during installation to guard against possible electrical shock which may be caused by the metallic sheath of the above-described cable engaging exposed electrical service wires or equipment in a plenum. Also, while the above-described cable meets flame spread requirements of the code, the snugness with which the metallic shield encloses the conductors prevents a charing of the conductor insulation that could effectively seal off a portion of the cable about the flame and reduce the evolution of smoke. One commercially available plastic material has been accepted as the covering material for plenum cable without the use of metal conduit, but it is relatively expensive and is difficult to process. The prior art also includes U.S. Pat. No. 3,425,865 which shows an electrical conductor covered successively with an inorganic, substantially flame-resistant material such as, for example, woven glass tape, a polyimide layer and a protective polyimide type braid coated with a polyimide finisher as an outer layer.
What is needed and what is not provided by prior art products is a cable which is covered with a material which is flame resistant and which has low smoke evolution. The sought after cable desirably is less costly than that of presently available products, is easy to process, and is available in sufficient quantities to satisfy escalating demands.